Attitude control system

ABSTRACT

A radar system for use on an aircraft or spacecraft for determining and controlling the attitude of the system antenna or of the craft, the system including an antenna unit producing four equiangularly spaced beams each of which is angularly offset by a substantial amount from a center reference axis, and means for applying radar pulse signals to each beam in succession and for deriving, from the received pulses returning from the ground, range and velocity information which is employed for orienting the antenna unit or the craft on which the system is mounted so as to impart a vertical orientation to the antenna system reference axis.

United States Patent Gaheen, Jr.

ATTITUDE CONTROL SYSTEM Inventor: Alfred F. Gaheen, Jr., Glen Burnie,Md.

Assignee: Westinghouse Electric Corporation, Pittsburgh, Pa.

Filed: March 24, 1969 Appl. No.: 809,779

U.S. Cl ..343/7.4, 343/7 ED, 343/73, 343/9 Int. Cl ..G0ls 9/02, GOls9/44 Field of Search ..343/7 ED, 7.3, 7.4, 9'

References Cited UNITED STATES PATENTS Badewitz ..343/7 ED RADAR CARRIER1 June 13, 1972 3,362,024 l/l968 Badewitz ..343/7 ED Primary Examiner-T.H. Tubbesing Attorney-F. H. Henson and E. P. Klipfel [57] ABSTRACT Aradar system for use on an aircraft or spacecraft for determining andcontrolling the attitude of the system antenna or of the craft, thesystem including an antenna unit producing four equiangularly spacedbeams each of which is angularly offset by a substantial amount from acenter reference axis, and means for applying radar pulse signals toeach beam in succession and for deriving, from the received pulsesreturning from the ground, range and velocity information which isemployed for orienting the antenna unit or the craft on which the systemis mounted so as to impart a vertical orientation to the antenna systemreference axis.

2 Claims, 2 Drawing Figures i [7 AVE I W GENERATOR l l (X-BAND vDISTRIBUTING AMPLIFIER a l "JB Q- BE L RANGE ii MIXER 21 41 DOPPLERiooPPugR 45 Pm: FREQUENCY 57 I FREQ 4s RANGE DETECTING lscANmNGaSCANNINGB I K FRoMRANGE' I RANGE TRACKING EEQ OUTPUTS 69$ GATE um'rRANGE I RANGE5 FROM 42 FROM4 RANGE RANGE FROM"l l l/FROM 44 DOPPLER 564| PITCH ROLL FREOUENCY/ ooRREcrcoRREcTl/u DETECTING umr umr FILTERFREQ. FfiFlinEeiz FREQ FREQ.

l FROM 4] t FROM 44 LATERAL LATERAL 7' VELOCITY VELOCITY 3 COMPUTINGCOMPUTING umr umr P'A'TENTEDJUM 13 m2 3. 670 334 sum 1 or 2 FIGJ.

mvsmon Alfred E Goheen,J r.

BY M

AT A ORNEY BACKGROUND OF THE INVENTION The present invention relates toa radar system, and particularly to a system for determining andcontrolling the attitude of the system antenna unit.

In the aircraft and spacecraft field, extensive use is made of radar forthe detection and identification of other craft and the identificationof underlying terrain. It has also been attempted to employ airborne orspaceborne radar systems to provide other information necessary for theproper operation of the craft, such information including the altitude,velocity and attitude of the craft relative to the ground. However, inorder for a radar system to provide accurate information regarding theseparameters, it is necessary that the antenna unit of such a system havean accurately determined attitude with respect to the ground becausevariations in the attitude of this unit will introduce errors into thevelocity and altitude indications provided by the system.

Various techniques have already been proposed for determining andcontrolling the attitude of such a unit. One such proposal involves theuse of the monopulse technique. This technique utilizes two closelyspaced beams whose areas of coverage partially overlap one another. Thebisector of the angle formed by the axes of the two beams is known asthe antenna boresight and the antenna unit is arranged to be oriented soas to cause this boresight to lie in a plane which is perpendicular bothto the ground being struck by the two beams and to the plane defined bythe beam axes. This is accomplished by transmitting equal-amplitudepulses along the two beams, determining the amplitudes of the receivedground return pulses associated with the two beams, and orienting theantenna unit until the amplitudes of the received pulses associated withthe two beams are equal.

While this system functions well when the craft on which it is mountedis over level, substantially homogeneous terrain, its accuracy isadversely affected, sometimes by a substantial amount, when other typesof terrain are encountered. For example, when the craft is situated sothat one beam strikes a water body while the other beam strikes a landmass, the difference in radar signal reflectivity between the twosurfaces being struck by the beams will result in an erroneousorientation of the antenna system boresight. Similarly, when the craftis over hilly or mountainous terrain, the inclination of the terrainwill result in an inaccurate orientation of the antenna boresight.

Moreover, since both antenna beams will be substantially perpendicularto the ground when the antenna system is properly oriented, this antennasystem could not itself be employed to provide useful informationregarding the horizontal velocity of the craft.

SUMMARY OF THE INVENTION It is a primary object of the present inventionto overcome these drawbacks and difficulties.

Another object of the invention is to permit an airborne or spaceborneradar system to provide highly accurate attitude information.

Still another object of the invention is to provide an attitude-sensingsystem which is not sensitive to the nature of the terrain encountered.

Still another object of the invention is to provide an attitude-sensingsystem which simultaneously presents altitude and relative velocityinformation.

Yet a further object of the invention is to provide a radar system whichacts to automatically maintain its antenna unit in the desiredorientation.

These and other objects according to the invention are achieved by theprovision of a novel pulse radar attitudesensing system for use in anaircraft or spacecraft. This system essentially includes antenna meansfor radiating radar pulse signals toward the ground and receiving theirreflections along two pairs of beams, radar pulse-generating means forgenerating a train of radar pulses and connected to the antenna means,and signal-processing means connected to the antenna means andpulse-generating means. The antenna means according to the invention areconstructed so that the beams of each pair are angularly offset from oneanother to define a large acute angle and so that the center axes of onebeam pair define a plane which intersects the plane defined by thecenter axes of the other beam pair along a line constituting thereference axis of the system. The radar pulse-generating means areconnected to the antenna means for causing each pulse to be radiatedalong only one beam thereof and for periodically shifting thetransmission of such pulses from one beam to another. Finally, thesignal-processing means are arranged for receiving information relatingto the transmission of radar pulses and the reception of theirreflection along at least one beam pair and for producing, on the basisof such information, an output representing the difference between themeasured distances to the ground along each beam of the pair, thisdifference being an indication of the inclination to the vertical of thereference axis of the system along the plane defined by the center axesof the beam pair.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective, pictorialview illustrating the basic principles of the present invention.

FIG. 2 is a block diagram of a radar circuit forming a part of apreferred embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The basis of theattitude-sensing and correction technique of the present invention isillustrated in FIG. 1 which is a pictorial view showing an aircraft Cflying over terrain T. This terrain may have any characteristics, i.e.it may be hilly, mountainous, partly dry land and partly water, but forthe sake of simplicity it is here shown as being flat.

The aircraft is equipped with an attitude-sensing system according tothe invention and this system includes an antenna unit which isconstructed to produce four discrete radar beams whose axes arerepresented by the broken lines b b b and b,. Each beam is preferablymade relatively narrow, i.e. to have the form of a pencil beam, and eachbeam strikes a small ground area, these areas being indicated as a a aand 11 respectively.

The attitude-sensing system carried by the craft further includes radarsignal-generating means connected to the antenna system for causing eachof the antenna beams to emit a signal whose return permits the rangefrom the craft to the ground along the axis of the beam to bedetermined. This could be accomplished in a straightforward manner byapplying a pulse signal to each beam and measuring the time required forthe transmitted beam to strike the ground and return to the craft. Toprevent confusion between the range signals associated with the variousbeams, a signal is applied to only one beam at a time and the periodbetween the application of a radar signal to one beam and theapplication of a radar signal to the next succeeding beam is selected topermit unambiguous reception at the maximum range which the system is toencounter.

The four beams 11,, k h and b., are composed of two pairs, one pair ofbeams, b, and b defining a plane which contains one axis of the craft,and the other pair of beams, b and b defining a plane which intersectsthe first-mentioned plane and which lies along a different axis of thecraft. For example, these two planes are generally perpendicular to oneanother and the plane defined by the first pair of beams extends alongthe longitudinal axis, or roll axis, of the craft, while the planedefined by the second pair of beams lies along the pitch axis of thecraft.

The line defined by the intersection of these two planes constitutes thereference axis of the antenna system and is indicated at v. The purposeof the attitude-sensing system according to the invention is to maintaina vertical orientation for this line, thus maintaining the antenna unitin a given orientation, or to determine the amount and direction of theinclination of this line with respect to the vertical, and thus toprovide an indication of the actual attitude of the antenna unit.

In accordance with the present invention, the orientation of thereference line along the longitudinal axis of the craft is firstdetermined by measuring the difference between the ranges indicated bythe radar signals associated with beams b and b After the range from thecraft to ground along each of the axis 12 and b;, has been detected bythe attitude-sensing system, the system locks onto these two rangeindications and then commences to also apply signals to the beams b andb, in altemation with the signals applied to beams b and h The rangeindications provided by beams b and b, are similarly compared to providean indication of the inclination of the antenna unit reference axis in adirection transverse to the craft.

If the antenna unit itself is to be maintained in the desiredorientation, the signal-processing circuitry of the attitudesensingsystem is arranged to provide error signals representing the differencebetween the ranges measured along beams b and b 3 and the differencebetween the ranges measured along beams b and b.,. The antenna unit ismounted on a platform which is capable of undergoing pivotal movementand the derived error signals are applied to suitable orienting motorsfor moving the platform into a position in which the reference axis ofthe antenna unit is vertical. Alternatively, the antenna unit could berigidly connected to the craft and the error signals could be applied toautomatically correct the attitude of the craft itself. Finally, theerror signals could simply be displayed to provide information forcorrecting the attitude of the craft or the antenna unit.

As is shown in FIG. I, the antenna unit is constructed so that theindividual beams are widely spaced angularly from one another, each beambeing inclined at an angle of 15 with respect to the reference axis v inpreferred embodiments of the invention. The large angle thus existingbetween each pair of beams is highly advantageous because it causes therange differential between the two beams to have a substantial valuewhen the reference axis of the antenna system is inclined, even by arelatively small amount, with respect to the vertical. On the otherhand, it is desired that this angle not be too large, because thegreater the inclination of the beams, the greater the required averagesignal power.

One consideration governing the minimum inclination angle of the beamsis the maximum initial inclination which the reference axis v could haveat the start of one attitude correcting operation. The inclination ofthe beams is preferably made sufficiently great to enable each beam tobe inclined in its intended direction with respect to vertical even whensuch a maximum inclination exists. For example, beam b should always bepointing ahead of the craft because if the inclination of axis v canbecome so great that the beam b could point toward the rear of thecraft, the difference between the ranges indicated by beams b and b 3would present an ambiguity with regard to the inclination which theyindicate.

The width of each beam determines essentially the area a of the groundwhich will be covered by the beam. In embodiments of the invention, thiswidth is in no way critical and can vary over a wide range. However, itis desired that the width not be too great since this would increase thepower which must be delivered to the beam to provide a return signal ofa particular amplitude and will reduce the accuracy of the resultingrange indication. On the other hand, it is desired that the width not betoo small because this would present certain antenna design problems andwould render the system too sensitive to small terrain irregularities.Thus, if the area a is sufficiently large, isolated hills or valleyswill not introduce any substantial error into the range indication. Inpractice, a beam subtending an angle of the order of 4 has been found tobe highly suitable.

The attitude-sensing system according to the invention is highlyadvantageous because the accuracy of its attitude indications are in noway adversely affected in those situations where one or several of thebeams are directed toward water bodies and the other beams are directedtoward land masses. This is true because the range indication producedby the system is not dependent on the reflection characteristics of thevarious surfaces.

Moreover, only very small inaccuracies are introduced into theattitude-determination when one or several of the beams encounters hillyor mountainous terrain because the range indication variation which thepresence of such terrain will produce will be quite small, particularlywhen the system is employed in high-altitude aircraft or spacecraft. Inthis connection it should be noted that, in such applications, errorsdue to the height of mountainous or hilly terrain are substantiallysmaller than are the errors produced in monopulse systems by theinclination of such terrain.

In further accordance with the invention, the attitudesensing systememploys range signals of a type which also provides velocityinformation, thus permitting both the altitude and the velocity of thecraft, with respect to two axes, to be determined. In order to assurethe presentation of accurate information, it is preferred that thevelocity indication only be provided after the range information hasbeen employed to bring the antenna unit to its desired orientation.

FIG. 2 is a circuit diagram of one radar signal processing circuit foruse in an attitude-sensing system according to the invention. Thecircuit shown is chosen only for exemplary purposes. It should beunderstood that many other arrangements of known units could be employedto attain the objectives of the present invention.

The circuit illustrated in FIG. 2 is composed firstly of an antenna unit1 composed of a gimballed support 5 and antenna elements ANT 1, ANT 2,ANT 3 and ANT 4, each of these elements producing a respective one ofthe beams b,to b, illustrated in FIG. 1. Each antenna element could beconstituted by one of four offset feeds associated with a singleparaboloidal dish. Such dish would form a part of the support 5.Alternatively, the antenna unit 1 could be composed of a Cassegrainreflector having four offset feeds, or by an all-electronic-scannedphased array or by an electronic lobe-switched phase array. Many othertypes of antenna arrangement could be employed, the present inventionbeing primarily concerned with the general relationship between theantenna beams rather than the specifics of the antenna structure. In anyevent, it is preferable that the antenna be of the type in which allfour beams share a common aperture.

The support element 5 is mounted on the craft and connected to have itsorientation controlled by a platform-orienting unit 6. The four antennaelements are electrically connected to an antenna-switching unit 8 whichcontrols the connection of each antenna element to the remainder of thecircuit. The operation of the antenna-switching unit is controlled by amaster control unit 10 in a manner to be described in detail below.

The signals which are to be radiated by the antenna elements along thefour beams are produced in the transmitter and modulator unit 11. Thisunit includes a radar carrier wave generator 13 producing a continuouscoherent X-band signal. This signal is sent to a mixer 15 which alsoreceives an [.F. signal delivered from an oscillator 17 via a gate 19.The output from the mixer 15, whose frequency is equal to the differencebetween the frequencies of the signals produced by the generator 13 andthe oscillator 17, is delivered to an amplifier 21 whose output iscontrolled by modulation pulses delivered by a pulse modulator 23connected to the amplifier 21.

The operation of gate 19 and pulse modulator 23 are controlled by radarsignal control pulses occurring at the system pulse repetition frequency(PRF), these pulses being delivered by the master control unit 10. Eachsuch pulse from the master control unit opens gate 19 and activatespulse modulator 23 so that at the output of amplifier 21 there willappear a pulse constituted by a signal burst whose frequency is equal tothe difference between the output frequency of the carrier wavegenerator and the output frequency of the I .F oscillator.

These signal pulses are delivered to a duplexer 25 which is connected toconvey pulses from the amplifier 21 to the antenna elements of unit 1,each pulse being delivered to a selected antenna element under thecontrol of switching unit 8. The duplexer 25 has another outputconnected to one input of a further mixer 31 for conducting thereto theground return pulse signals received by the antenna elements. Mixer 31is also connected to carrier wave generator 13 for receiving the X-bandfrequency of that generator.

The output signal from mixer 31 will have a frequency equal to thedifference between the frequencies of the signals applied to its twoinputs. Thus, during the period when a received echo pulse signal isbeing delivered to mixer 31, the mixer output signal will be at afrequency equal to the 1.1-. frequency (X-Band and frequency minustransmitted frequency) PLUS A Doppler shift frequency proportional tothe velocity of the craft in the direction of the axis of the radar beamrelative to the ground area struck by the beam.

The output from mixer 31 is delivered to the input of an LP. amplifier33 gain is controlled by plus a unit 35. The passband of the LF.amplifier 33 is centered on the frequency of the output of LF.oscillator 17 so that this amplifier will only be responsive to theoutput from mixer 31 when a reflected pulse signal is being received.The A.G.C. unit 35 is simply provided to cause the amplifier 33 toproduce output pulses of uniform amplitude in response to the receivedreflected signals associated with all four beams. This signal amplitudeuniformity assures that the elements provided for processing the signalsassociated with the four beams will receive comparable signals.

The output from l.F. amplifier 33 is delivered to a distributingamplifier 38 having four output channels, Ch. 1, Ch. 2, Ch. 3 and Ch. 4.Distributing amplifier 38 is provided with a control input connected tomaster control unit for receiving control signals that determine whichoutput channel will receive each pulse delivered by amplifier 33. Thecontrol of distributing amplifier 38 is synchronized with that ofantennaswitching unit 8 in such a manner that the reflected pulsesignals received by each antenna element will be directed to onecorresponding output channel, the reference numeral of each such channelcorresponding to the reference numeral of its associated antennaelement.

Each distributing amplifier output channel is connected to a respectiveone of four identical signal-extraction circuits 41, 42, 43 and 44. Onlythe circuit 41 is shown in detail, each of the otherinformation-extraction circuits being identically constructed.

Circuit 41 includes a mixer 45 having one input connected todistributing amplifier output channel Ch. 1 and its other inputconnected to the output of a Doppler frequency scanning and trackingunit 47. The output from mixer 45 is connected to the input of a rangegate 46 which is provided to determine the length of the time intervalbetween the transmission of a signal pulse by elements ANT. 1 and thereception thereby of the returning reflection pulse, this time intervalbeing proportional to the distance, or range, between the craft and theground area a,. The opening of range gate 46 is controlled by a rangescanning and tracking unit 48 which is connected to receive the outputfrom range gate 46 and which hence forms a feedback loop for the rangegate.

Unit 48 is also connected to receive the radar control pulse signalsproduced at the pulse repetition frequency by master control unit 10 andfunctions, in a manner to be described in greater detail below, to bringthe opening of range gate 46 into time coincidence with the appearanceof a received signal pulse at output Ch. 1 of amplifier 38. Once such acoincidence has been achieved, unit 48 produces a range signal at itsRange Output representing the time interval between the transmission ofa pulse by element ANT. 1 and its reception of the return pulse. Theoutput from range gate 46 is connected to a corresponding input of theA.G.C. unit 35 to adjust the amplification imparted by amplifier 33 tothe return signals from antenna element ANT. 1.

from master control unit 10, such opening being eflected during eachinterval when signals associated with amplifier output Ch. 1 are beingprocessed. During this interval, any signals appearing at the output ofrange gate 46 are transmitted via gate 51 to a Dopplerfrequency-detecting filter bank 56 which detects the frequency of thecarrier wave of the received pulse. The output from filter bank 56 isconducted to the Doppler frequency scanning and tracking unit 47 which,in responseto output signals therefrom, applies to mixer 45 a signalwhose frequency bears a predetermined relation with the frequency of theLF. oscillation appearing at output Ch. 1 of amplifier 38. When thisrelationship is established, the output from mixer 45 will be at apredetermined frequency and the output from unit 47 will have afrequency proportional to the Doppler shift frequency of the returnsignal received by element.

ANT. 1. Since the value of this frequency will be proportional to boththe velocity of the craft relative to the ground area struck by theradar signal in the direction of the associated beam and to theorientation of the antenna platform, this frequency will constitute anaccurate indication of one component of the craft velocity only when theantenna platform is properly oriented wit its reference axis v (FIG. 1)extending vertically. Therefore, master control unit 10 is preferablyarranged to open gate 51 only after the platform has been so oriented.However, the system could also be arranged so that the frequency unitbegins tracking before the range unit to assure proper acquisition bythe frequency unit.

Each of the other information extraction units 42, 43 and 44 produces acorresponding range output and frequency output and is associated with acorresponding gate 52, 53, or 54. Since radar pulses are transmitted byeach of the antenna elements in sequence, and the received return signalassociated with each antenna element is delivered to a correspondinginformation extraction unit before a radar pulse signal is applied tothe next succeeding antenna element, only one information extractioncircuit receives a signal during any given radar pulse repetitioninterval so that it is possible for two information extraction units toshare a common Doppler frequencydetecting filter bank. Thus, in theillustrated embodiment, information extraction unit 42 shares filterbank 56 with unit 41, while units 43 and 44 share a similar filter bank57.

The range output signals from units 41 and 43, which indicate the rangesalong beams b and b respectively, are delivered to a pitch correctionunit 61, while the range output signals from units 42 and 44,corresponding to the ranges along beams b and b,, are delivered to aroll correction unit 63. The outputs from these correction units'aredelivered to platform-orienting unit 6 which pivots the platform-5 inorder to give it the desired orientation. The output from pitchcorrection unit 61 controls the pivoting of platform 5 in a.

direction perpendicular to the craft pitch axis in such a manner as tocause the range outputs from units 41 and 43 to approach equality.Similarly, the output from roll correction unit 63 causes the platform 5to be pivoted in a direction perpendicular to the craft roll axis insuch a manner as to create equality between the range outputs from units42 and 44.

If desired, the platform-orienting unit may be constructed to provide anindication of the orientation of the platform 5 relative to the craft toprovide information as to the actual craft 7 orientation once theantenna has been oriented.

After the antenna platform 5 has been oriented to have the proper pitch,a signal is sent by pitch correction unit 61 to master control unit 10and after the platform has been given the desired roll orientation, acomparable signal is delivered by roll correction unit 63 to mastercontrol unit 10. Upon receipt of these signals, unit 10 acts tosubsequently open gates 51 to 54 at the appropriate times to permit theproduction of frequency output signals representing the Dopplerfrequency content of each received return pulse.

The frequency output signals from units 41 and 43 are delivered to aforward velocity-computing unit 71, while the corresponding outputs fromunits 42 and 44 are delivered to a lateral velocity computing unit 73.These velocity computing units function in a known manner to convert theDoppler frequency information associated with the received returnsignals into indications of the forward velocity component V; of thecraft in the plane defined by beams b, and b and the lateral velocitycomponent V of the craft along the plane defined by beams b and b All ofthe units illustrated in FIG. 2 are well known and could easily befabricated by workers skilled in the art. Moreover, there are many othercircuit arrangements which could be devised by those skilled in the artto derive the desired information from radar pulse signals of the typedescribed.

In the illustrated circuit, the master control unit controls theantenna-switching unit 8, the distributing amplifier 38, the A.G.C. unit35 and the gates 51 to 54in synchronism so as to cause each radar pulsesignal to be delivered to one antenna element and the correspondingreceived return signal to be processed by one associated informationextraction unit so that the radar signals associated with each antennaelement are delivered to one corresponding information extraction unit.

In operation, it is preferred that the system first begin tracking theranges associated with beams b and b before it begins processing signalsassociated with beams b and b.,. To this end, the master control unit 10is arranged to deliver radar pulses only to elements ANT.1 and ANT.3 andto distribute the received return pulses only to units 41 and 43 untilthe desired tracking with respect to the beams is achieved. Theachievement of the desired tracking could be indicated by the deliveryof the signal from pitch correction unit 61 to master control unit 10.After the desired tracking has begun, the master control unit switchesinto an operating mode in which the antenna elements are activated inthe sequence 1, 3, 2, 4, etc.

Range scanning and tracking unit 48 is operated in a known manner tofirst open range gate 46 at a succession of instants after theoccurrence of the transmitted radar pulse until one gating instant isencountered when a signal appears at the output of the range gate. Theoccurrence of this signal is detected by unit 48 and causes the unit toswitch into a tracking mode in which it causes range gate 46 to open atthe corresponding instant during each successive tracking interval andvaries the occurrence of this instant slightly in response to subsequentvariations occurring from one scanning interval to the next. The Dopplerfrequency-scanning and tracking unit 47 operates in a comparable manner.

If an attitude-sensing system according to the invention is employed ina spacecraft, the antenna platform 5 could be rigidly connected to thecraft and the pitch and roll correction unit connected to operateappropriate attitude control jets provided on the craft so that thesystem would act to properly orient the entire craft rather than justthe antenna platform.

Alternatively, for either aircraft or spacecraft applications, theattitude-sensing system could simply provide indications of the variousrange values on the basis of which an operator could adjust theorientation of the craft or the antenna platform.

In one exemplary embodiment of the invention, a PRF of 210 pulses/secondwas employed to provide an unambiguous range interval of 390 nauticalmiles and each transmitted pulse had a duration of 75 microseconds. Atthis pulse repetition frequency, and when all four beams are beingemployed, each beam is pulsed at a rate of 52.5 times per second. Thesystem transmitter was constructed to produce an X-band output at afrequency of 10 GHz and an LP. output at a frequency of 30 MHZ.

Systems according to the invention could be constructed in astraightforward manner to also yield altitude and vertical velocityinformation on the basis of the range and velocity information providedby all four beams.

It is also a simple matter to produce an indication of the resultanthorizontal velocity of the craft on the basis of the forward velocityand lateral velocity component indications.

It will be understood that the above description of the presentapplication is susceptible to various modifications, changes andadaptations.

I claim:

1. A pulse radar attitude sensing system for use in an aircraft orspacecraft, comprising in combination:

a. antenna means for radiating radar pulse signals toward the ground andreceiving their reflections along two pairs of beams, with the beams ofeach pair being angularly spaced from one another to define a largeacute angle and the center axis of one pair of beams defining alongitudinal plane which intersects the plane defined by the center axisof the other pair of beams along a line constituting the reference axisof said system, said beams in said longitudinal plane being directedfore and aft, respectively, of said reference axis, the beams of saidother pair being directed transversely of said longitudinal plane;

b. radar pulse generating means for generating a train of radar pulsesand commutation means connected between said generating means and saidantenna means for causing said radar pulses to be sequentially routed tosaid antenna means to sequentially generate said angularly spaced beams;

c. signal processing means connected to said antenna means for receivingecho pulses resulting from transmission of said radar pulses;

d. means for determining the transient time between the transmission andreception of said pulses along each of said beams to determine altitudeinformation;

e. means for determining the Doppler frequency shift in the receivedecho pulses for determining the velocity components with respect to theground along orthogonal axes,

f. antenna attitude control means responsive to the output signals fromsaid signal processing means for adjusting the attitude of said antennameans in accordance with information included in said echo signals;

g. said commutator means being constructed to connect said antenna togenerate in sequential order the forwardly directed beam in saidlongitudinal plane, then the other beam in said longitudinal plane, thenthe first one and then the other of said transverse beams;

h. and gate means responsive to received echo signals for controllingsaid commutation means so that the received echo signal associated witheach antenna element is received and processed before a radar pulsesignal is applied to said antenna means to generate the next beam.

2. An arrangement as defined in claim 1 wherein each said beam lies atan angle of 15 with respect to said reference axis of said system.

1. A pulse radar attitude sensing system for use in an aircraft orspacecraft, comprising in combination: a. antenna means for radiatingradar pulse signals toward the ground and receiving their reflectionsalong two pairs of beams, with the beams of each pair being angularlyspaced from one another to define a large acute angle and the centeraxis of one pair of beams defining a longitudinal plane which intersectsthe plane defined by the center axis of the other pair of beams along aline constituting the reference axis of said system, said beams in saidlongitudinal plane being directed fore and aft, respectively, of saidreference axis, the beams of said other pair being directed transverselyof said longitudinal plane; b. radar pulse generating means forgenerating a train of radar pulses and commutation means connectedbetween said generating means and said antenna means for causing saidradar pulses to be sequentially routed to said antenna means tosequentially generate said angularly spaced beams; c. signal processingmeans connected to said antenna means for receiving echo pulsesresulting from transmission of said radar pulses; d. means fordetermining the transient time between the transmission and reception ofsaid pulses along each of said beams to determine altitude information;e. means for determining the Doppler frequency shift in the receivedecho pulses for determining the velocity components with respect to theground along orthogonal axes, f. antenna attitude control meansresponsive to the output signals from said signal processing means foradjusting the attitude of said antenna means in accordance withinformation included in said echo signals; g. said commutator meansbeing constructed to connect said antenna to generate in sequentialorder the forwardly directed beam in said longitudinal plane, then theother beam in said longitudinal plane, then the first one and then theother of said transverse beams; h. and gate means responsive to receivedecho signals for controlling said commutation means so that the receivedecho signal associated with each antenna element is received andprocessed before a radar pulse signal is applied to said antenna meansto generate the next beam.
 2. An arrangement as defined in claim 1wherein each said beam lies at an angle of 15* with respect to saidreference axis of said system.